20 years ago today, the very first episode of Star Trek: Deep Space Nine aired on American television. And Star Trek would never be the same again — but Deep Space Nine also had a lasting impact on all genre television. The grittiest of Star Trek series helped to give us a lot of the things we take most for granted on science fiction and fantasy television today — and here are just a few of the ways.

Those long, sweeping arcs. It seems like a weird thing to mention now, because every random cop show or space adventure has storylines that carry on from episode to episode, more like serialized novels than collections of self-contained stories. But back when Deep Space Nine started, the idea of following "arcs," especially ones that went on for more than one season, was still more unusual on TV. For a Star Trek show, especially, it was considered weird to have so many continuing storylines.

The Enterprise, like I said earlier, could pull up to a planet and have an episode and keep going. With Deep Space Nine, anything that took place on the station, well guess what? Next week you are still on the station. And Bajor is not going anywhere. So really you had to keep playing those stories. You couldn't make a big change in Bajor's political structure in one week and then ignore it then next. You had to keep it going. Kira's story with his relationship with Bajorans had to keep evolving and so did Sisko's and they had a long-term mission. They had a mission about Bajor into the Federation. That alone meant that it was going to be serialized at least on that front.

The good terrorists In the past decade, we've seen many science fiction shows which tried to show terrorists and resistance fighters in a sympathetic or complicated light — and this arguably started with DS9. Quite possibly the most sympathetic character on Deep Space Nine is Major Kira, who was a "resistance fighter" on Bajor when the Cardassians occupied the planet. We hear endlessly about how she planned raids and bombings against the occupiers. We also hear the Cardassian side, about how they thought they were doing the right thing occupying Bajor, and how Gul Dukat, the leader of the occupation, wanted to win the Bajorans' love and respect, but they kept pushing him with their unethical terror tactics. Towards the end of the show, there's a wonderful reversal where the Cardassians are themselves occupied by the Dominion (because Dukat gave away the store) and Kira comes in to teach her former oppressors the tactics she used against them.

The enemy among us. The Dominion, DS9's evil empire, is run by "changelings," shapeshifters who can look like anyone or anything. (Except for Odo, our friendly shapeshifter, who is a bit pants.)

Honestly, DS9 didn't do enough with the "shapeshifter" plot, because if anyone can be a shapeshifter, you open the door for endless paranoia. Commander Sisko could be replaced by a shapeshifter at any moment, and so could Dax or Kira. (And Bashir actually was replaced by a Changeling for quite some time.) The show only follows this plot to its logical conclusion a couple of times — once when the changelings infiltrate Starfleet Headquarters in "Home Front," and Starfleet imposes martial law. (Sisko finally realizes what a blunder this is, and there's a great speech about how if the changelings want the humans to lose their freedoms, they'll have to take them away themselves.) And the other time, a leading Klingon is replaced by a changeling and nearly starts a war. But for the most part, the "evil shapeshifters" plot goes on the back burner an awful lot.

Divided loyalties. In DS9, Odo becomes one of the most compelling characters on the show because he discovers he's actually a member of the evil changeling race, which oppresses the Gamma quadrant and wants to take over the Alpha quadrant. And this show also helped to introduce the Maquis, the Federation members who go rogue to fight the Cardassians for some colonies in Cardassian territory — and arguably did more with the Maquis than Voyager, which featured Maquis crew members. Plus there's always Worf, who's torn as usual between his Klingon and Federation allegiances.

The mismatched romantic pairings. We're used to seeing some really off-kilter, star-crossed love stories on television these days. If you have a Vampire Slayer, she's bound to fall for a vampire. But DS9 gave us some of the weirdest romances we had seen on television at the time.

The love story between Odo and Kira is one of the few really compelling romantic sagas in Trek — unless you think Troi/Riker is full of awesome. But it does have some problems, like Kira's decision to date Odo comes out of nowhere. And I was annoyed with Odo's "changeling bros before hos" moment in the finale, where he ditches Kira to go off with his own people. But still — Odo pines for her for so long, and they finally get together, and he turns into mist in that one episode and swirls around her. It's just too radical. And I actually loved the Dax/Worf pairing as well.

A serious focus on religion. When DS9 first aired, the trope among science fiction TV shows was to show false gods and superstitious people who worshiped technology in the mistaken belief that it was magic. We could be here all day listing every fake God James Kirk debunked, and don't get us started on Doctor Who. But DS9 took a different tack — taking religions seriously as cultural and social forces.

In fact, Deep Space Nine has religion built into its DNA at so many levels, it's practically a religious allegory with space-opera trappings. Let's review: there are some mysterious aliens who exist out of time, whom the Bajorans worship as "the Prophets." The Prophets anoint Benjamin Sisko as their "Emissary," and it turns out they possessed his mother and arranged his birth. He's supposed to help them fight their evil counterparts, the Pagh Wraiths. Meanwhile, the Dominion worships its leaders, the changelings, as gods. It's interesting: in DS9, the non-Bajoran characters refer to the Prophets as "the wormhole aliens" — until somewhere in season three or four. You never hear the phrase "wormhole aliens" (I think) in the last few seasons of the show. And there's less and less doubt that Sisko has a holy mission on the Prophets' behalf. Sadly, it doesn't end that well — there's one episode where Sisko's son gets possessed and has to fight a holy battle, and it's both confusing and boring. And then in the end, the giant battle between the good gods and the evil gods turns out to be a matter of jumping off a cliff. And it's pretty clear the changelings aren't gods — they're just slimy shapeshifters who have tweaked their slaves' genomes or gotten them hooked on drugs, so they'll be worshiped forever.

Those are just a few of the ways that Deep Space Nine helped to drag science fiction television, kicking and screaming, into maturity. What's your favorite Deep Space Nine memory? (And for further reading, here are 10 things you probably didn't know about DS9.)

This io9 flashback originally appeared, in very different form, back in 2008.